The present invention relates generally to consumable welding wires or electrodes for the welding of intermetallic alloys such as nickel aluminides, nickel-iron aluminides, iron aluminides, and titanium aluminides and more particularly to the fabrication of such consumable wires which comprise a two-component, clad structure defined by a core and a sheath, one of which is formed of the primary intermetallic alloying constituent(s) except for the aluminum while the other is formed of the aluminum.
Intermetallic alloys such as provided by nickel aluminides, nickel-iron aluminides, iron aluminides, and titanium aluminides are being increasingly utilized in engineering structures in place of stainless steels and other metals or alloys these intermetallic alloys are less expensive and possess highly desirable mechanical properties at the elevated temperatures. Developments in these intermetallic alloys have resulted in significant improvements in the mechanical properties so as to even further increase their suitability for use in engineering structures. For example, in U.S. Pat. No. 4,711,761 to C. T. Liu et al, it was pointed out that the addition of boron (about 0.005-0.5 wt %) had been previously used in nickel aluminide alloys to reduce the brittleness of these alloys. This patent discloses that the strength of these boron-containing nickel-aluminide alloys, when incorporating about 9-11.5 wt % aluminum, is further improved by the additions of about 6-16 wt % iron which provides a desirable solid solution hardening effect. Also, in this patent it was disclosed that various combinations of other alloying constituents such as manganese (about 0.05-1.0 wt %), titanium (about 0.5 wt %), and niobium (about 1.3 wt %) are useful for increasing the fabricatability of these iron-containing nickel aluminides or nickel-iron aluminides. A further development in the evolution of nickel-aluminide and nickel-iron aluminide alloys is reported in U.S. Pat. No. 4,731,221 to C. T. Liu where chromium additions of about 1.5-8 at. % (about 1.4 to 7.9 wt %) provides for a substantial increase in the ductility of the nickel-aluminides and the nickel-iron aluminides at intermediate temperatures in the range of about 400.degree. to 800.degree. C. as well as improving creep oxidation resistance of such alloys. Also, the addition of about 0.2-1.5 at. % (about 0.3 to 5.0 wt %) of a Group IVB element, namely, hafnium, zirconium, or mixtures thereof in such alloys with or without the chromium addition provide an increase in high temperature strength. Cerium additions, like iron, was found to increase the ductility and the fabricability of these intermetallic alloys.
The mechanical and other properties of iron-aluminide alloys have also been significantly improved with early developments described in U.S. Pat. No. 3,026,197 to J. H. Schramm where the addition of zirconium and boron was employed to refine the grain structure in iron-aluminide alloys. A more recent improvement in these iron-aluminum alloys is described in U.S. Pat. No. 4,961,903 to C. G. McKamey et al where iron-aluminum alloys containing about 20 to 30 at. % (about 10.8 to 16.9 wt %) aluminum are provided with improved room temperature ductility, increased high temperature strength, and reduced susceptibility to corrosion by the additions of about 0.5 to 10 at. % (about 0.52 to 10.86 wt %) chromium, up to about 2.0 at. % (4.01 wt %) molybdenum, up to about 1.0 at. % (1.94 wt %) niobium, up to about 0.5 at. % (0.95 wt %) zirconium, 0.02 to about 0.3 at. % or 0.8 wt % boron, and/or carbon, up to 0.5 at. % (0.53 wt %) vanadium, and up to 0.1 at. % (0.18 wt %) yttrium. In U.S. Pat. No. 5,084,109 to V. K. Sikka et al, iron-aluminide alloys containing about 25 to 31 at. % (about 14 to 18 wt %) aluminum, and preferably including up to a total of about 12 at. % of an element or a combination of elements selected from chromium, niobium, zirconium, molybdenum, boron, and carbon is used to provide the intermetallic alloy with increased room temperature ductility and high temperature strength when the alloy is thermomechanically worked. Another recent development in the improvement of the mechanical properties of iron-aluminide alloys is described in commonly assigned U.S. patent application Ser. No. 07/904/802, filed Jun. 26, 1992, in the name of V. K. Sikka et al, where iron-aluminide alloys containing 8-9.5 wt % aluminum are provided with room temperature ductilities greater than 20%. The addition of an effective amount of chromium ranging from more than incidental impurities up to about 7 wt % was used to promote corrosion resistance of the alloy to aqueous solutions while the addition of about 4 wt % molybdenum was used to promote solution hardening as well as resistance of the alloy to corrosion in solutions containing chloride. A carbide former, preferably zirconium about 0.15-0.25 wt %, was combined with up to about 0.05 wt % carbon in the alloy for controlling grain growth in the iron-aluminide alloys at elevated temperatures.
Titanium aluminides provide relatively light-weight intermetallic alloys which possess high strength at elevated temperatures so as to render them particularly suitable for use in automotive, aeronautical and space applications. The titanium aluminides are usually of alpha (Ti.sub.3 Al) or gamma (TiAl) type alloys. Typical compositions for the Ti.sub.3 Al alloy is a titanium base with about 22 to 35 at % aluminum, about 10 to 24 at % niobium for improving room temperature ductility of the alloy, about 3 at % vanadium, and about 0.5 to 1.0 at % molybdenum. The TiAl alloy typically comprises a titanium base with about 48 to 55 at % aluminum and about 2 to 4 at % of other alloying elements for improving room temperature ductility.
Previously known developments in nickel-aluminide alloys, nickel-iron aluminides, iron-aluminide, and titanium aluminide alloys such as generally described above and as disclosed in the aforementioned patents and the commonly assigned U.S. patent application, as well as in the publications and patents cited therein, provide these intermetallic alloys with highly desirable mechanical properties including high temperature strength, corrosion resistance, and good room temperature ductility. However, a significant drawback to the use of these intermetallic alloys in engineering applications which require the welding together of various structural components for forming engineering structures has not yet been satisfactorily addressed. The utilization of intermetallic alloys of the type described above in engineering structures is critically dependent upon the use of welding as a primary fabrication technique. This problem or drawback associated with the welding of intermetallic alloys is due to the difficulties encountered in processing the intermetallic alloys into consumable welding rods or wires by employing known metal working techniques usually practiced at elevated temperatures since these intermetallic alloys exhibit high temperature strength and limited ductility at such conventional metal working temperatures.